Re: [PATCH v8 06/10] mm: thp: check pmd migration entry in common path

[Naoya Horiguchi]

On Sat, Jul 01, 2017 at 09:40:04AM -0400, Zi Yan wrote:
> From: Zi Yan <zi.yan@xxxxxxxxxxxxxx>
> 
> If one of callers of page migration starts to handle thp,
> memory management code start to see pmd migration entry, so we need
> to prepare for it before enabling. This patch changes various code
> point which checks the status of given pmds in order to prevent race
> between thp migration and the pmd-related works.
> 
> ChangeLog v1 -> v2:
> - introduce pmd_related() (I know the naming is not good, but can't
>   think up no better name. Any suggesntion is welcomed.)
> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> 
> ChangeLog v2 -> v3:
> - add is_swap_pmd()
> - a pmd entry should be pmd pointing to pte pages, is_swap_pmd(),
>   pmd_trans_huge(), pmd_devmap(), or pmd_none()
> - pmd_none_or_trans_huge_or_clear_bad() and pmd_trans_unstable() return
>   true on pmd_migration_entry, so that migration entries are not
>   treated as pmd page table entries.
> 
> ChangeLog v4 -> v5:
> - add explanation in pmd_none_or_trans_huge_or_clear_bad() to state
>   the equivalence of !pmd_present() and is_pmd_migration_entry()
> - fix migration entry wait deadlock code (from v1) in follow_page_mask()
> - remove unnecessary code (from v1) in follow_trans_huge_pmd()
> - use is_swap_pmd() instead of !pmd_present() for pmd migration entry,
>   so it will not be confused with pmd_none()
> - change author information
> 
> ChangeLog v5 -> v7
> - use macro to disable the code when thp migration is not enabled
> 
> ChangeLog v7 -> v8
> - remove not used code in do_huge_pmd_wp_page()
> - copy the comment from change_pte_range() on downgrading
>   write migration entry to read to change_huge_pmd()
> 
> Signed-off-by: Zi Yan <zi.yan@xxxxxxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/mm/gup.c             |  7 +++--
>  fs/proc/task_mmu.c            | 33 ++++++++++++++-------
>  include/asm-generic/pgtable.h | 17 ++++++++++-
>  include/linux/huge_mm.h       | 14 +++++++--
>  mm/gup.c                      | 22 ++++++++++++--
>  mm/huge_memory.c              | 67 +++++++++++++++++++++++++++++++++++++++----
>  mm/memcontrol.c               |  5 ++++
>  mm/memory.c                   | 12 ++++++--
>  mm/mprotect.c                 |  4 +--
>  mm/mremap.c                   |  2 +-
>  10 files changed, 154 insertions(+), 29 deletions(-)
> 
> diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
> index 456dfdfd2249..096bbcc801e6 100644
> --- a/arch/x86/mm/gup.c
> +++ b/arch/x86/mm/gup.c
> @@ -9,6 +9,7 @@
>  #include <linux/vmstat.h>
>  #include <linux/highmem.h>
>  #include <linux/swap.h>
> +#include <linux/swapops.h>
>  #include <linux/memremap.h>
>  
>  #include <asm/mmu_context.h>
> @@ -243,9 +244,11 @@ static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
>  		pmd_t pmd = *pmdp;
>  
>  		next = pmd_addr_end(addr, end);
> -		if (pmd_none(pmd))
> +		if (!pmd_present(pmd)) {
> +			VM_BUG_ON(is_swap_pmd(pmd) && IS_ENABLED(CONFIG_MIGRATION) &&
> +					  !is_pmd_migration_entry(pmd));

This VM_BUG_ON() triggers when gup is called on hugetlb hwpoison entry.
I think that in such case kernel falls into the gup slow path, and
a page fault in follow_hugetlb_page() can properly report the error to
affected processes, so no need to alarm with BUG_ON.

Could you make this VM_BUG_ON more specific, or just remove it?

Thanks,
Naoya Horiguchi

>  			return 0;
> -		if (unlikely(pmd_large(pmd) || !pmd_present(pmd))) {
> +		} else if (unlikely(pmd_large(pmd))) {
>  			/*
>  			 * NUMA hinting faults need to be handled in the GUP
>  			 * slowpath for accounting purposes and so that they